add a test case for syslog over UDP

The new capture contains two datagrams produced by logger utility
(syslog default timestamp format) and two datagrams produced by rsyslog
(RFC3339 timestamp format). One of the rsyslog datagrams has priority
consisting of two figures and the other -- of one figure.
fix parsing of syslog priority (GH #264)

...the code

* first looks for '<' and advances to the next character,
* it looks for a number between 0 and 9 and advances to the next
  character, (it finds 7)
* it looks for '>' and advances to the next character,
* it looks for a number between 0 and 9 and advances to the next
  character, (it finds the '2' of '2010')

=> result: prio is 72 instead of 7.

The code that checks if the character is '>' should be outside the loop
that checks if the character is a number. The attached patch moves this
check.
syslog: refine the decoder

Switch to TCHECK2() and improve format checks further. Remove some
excess code, an unused #include, some trailing whitespace and a few
empty lines.